Catalysts and methods for hydrogenating chemical compounds containing ethylenic and/or aromatic unsaturation are known and are described in, for example, U.S. Pat. Nos. 3,415,759 and 5,057,582. The older prior art describes the use of heterogeneous catalysts such as nickel on supports such as kieselguhr (diatomaceous earth) and Raney nickel. More recently, the use of homogeneous catalyst systems have been reported, especially when selective hydrogenation as between ethylenic and aromatic unsaturation was desired.
Catalysts useful for selective hydrogenation are made by contacting one or more Group VIII metal carboxylates (carboxylates of Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt) with one or more aluminum alkyls. Such catalysts can produce excellent results in that they selectively hydrogenate ethylenic unsaturation to a high degree while not hydrogenating aromatic unsaturation.
Hydrogenation of block copolymers has been studied since the 1960s. Much work has been focused on the use of homogeneous Ziegler-type catalysts prepared by alkyl aluminum reduction of various Group VIII metal carboxylates. In this large body of work, nickel and cobalt have often been compared and reported to have similar activity.
In the 1960s and 1970s the reaction kinetics in the field of anionic polymerization that were available to researchers were almost exclusively done by hand and reactions were deemed to be “complete” simply by time. While this practice works and is capable of producing the desired block copolymers, variation in temperature profiles from batch to batch resulted in varying levels of “die-out”
Anionic polymerization is said to produce living polymer chains. “Anionic polymerization causes the formation of so-called ‘living polymers’ because the ionic sites remain active.” Ulrich, Introduction to Industrial Polymers, p. 48 (1982). “Die-out” occurs when the living chains lose or otherwise have the ionic site become inactive. In cases of die-out, the chains are terminated prematurely and this can have several undesirable consequences such as the production of a wide molecular weight range.
Conventional computerized kinetics calculate monomer conversion as a function of the temperature profile and monomer concentration so that die-out is minimized. Improvements in gel permeation chromatography technology have allowed the further refining and fine tuning of the polymerization kinetics since the ability to detect and quantify subtle differences between polymers produced under different conditions has improved.
In the early development of the selective hydrogenation catalysts, it was very common to use a large excess of alcohol to terminate the polymerization. Living polymers had the potential to crosslink while awaiting hydrogenation and it was standard procedure to make certain that the polymer was completely terminated. It was not uncommon in lithium activated polymerizations to use alcohol to lithium molar ratios of 1.3 and above, even up to 2.0.
It would be desirable in the art of preparing hydrogenated block copolymers of vinyl aromatic hydrocarbons and conjugated dienes to selectively hydrogenate residual alkyl non-aromatic unsaturation. It would be particularly desirable in the art to perform such selective hydrogenation using a catalyst under conditions such that the hydrogenation is done more quickly and effectively. It would be more desirable still if the catalysts and conditions needed for the more effective hydrogenation were compatible with conventional equipment and processes for such hydrogenation.